1) Field of the Invention
The present invention relates to an optical device used in an optical communication system such as wavelength division multiplex transmission, and particularly to an optical device in which an optical waveguide chip is contained in a package and a manufacturing method thereof.
2) Description of the Related Art
With widespread use of the Internet, mobile phones, and advanced features of terminal equipment, increasing transmission capacity is a mandatory challenge. Thus, communication networks are becoming increasingly dependent on optical fibers capable of high-speed, large-capacity transmission, and thus construction and maintenance of optical communication networks using WDM (Wavelength Division Multiplexing) is urgently needed.
FIG. 13 is a block diagram exemplifying an optical transmitter performing transmission of optical signals in a WDM transmission system. Numeral 101 denotes an optical fiber outputting optical signals, numeral 102 denotes an optical modulator connected to the optical fiber 101, and numeral 103 denotes a light source part comprised of a semiconductor laser 103a optically connected to the optical modulator 102 and a temperature control device 103b controlling temperature of the semiconductor laser 103a. Further, numeral 104 denotes a driving part comprised of an LD driving circuit 104a and an LD temperature control circuit 104b, and numeral 105 denotes an optical modulator driving circuit.
The semiconductor laser (made of a laser diode, for example, and called “LD” below) 103a used as a light source is driven by the LD driving circuit 104a whose temperature is controlled by the temperature control device 103b receiving a control signal from the LD temperature control circuit 104b to control optical power and oscillation wavelengths to fixed values.
The optical modulator 102 is an external optical modulator and is constructed of an optical waveguide chip using a substrate made of material having an electro-optical effect such as LiNbO3 (called “LN” below) Electrodes are provided near the optical waveguide and an output light (continuous light) from an LD is modulated in intensity by an electric signal applied to the electrodes to generate an optical signal in pulse forms before being transmitted to the optical fiber 101.
Miniaturization, advanced features, and cost reduction of such an optical transmitter described above have been strongly demanded with widespread use of WDM communication in recent years, and a demand for an optical modulator constituting the optical transmitter is no exception.
In such a backdrop, realization of (a) miniaturization by making an optical fiber shorter and (b) advanced features by making an optical fiber multi-core (multi-channel) concerning particularly a connection structure of an optical fiber has been demanded to achieve miniaturization and advanced features of optical waveguide devices including optical modulators. Cost reduction of optical waveguide devices by achieving simplification of assembly has also been keenly called for.
A configuration of a combined system of an optical waveguide and optical fiber in optical waveguide devices including optical modulators is exemplified for example by Patent Document 1 shown below. FIG. 14 shows an outline of a configuration example of an optical waveguide device 110 described in Patent Document 1.
As shown in FIG. 14, an opening 114 for passing an optical fiber 113 to the outside is provided on a sidewall of a package 112 containing an optical waveguide chip 111, and a pipe 115 is fixed airtightly around the opening 114. Then, the optical waveguide chip 111 and the optical fiber 113 are fixed by an adhesive via a first ferrule 116 and the optical fiber 113 is airtightly fixed to a second ferrule 117 at an introduction location to the inside of the package 112 through the pipe 115. Further, the second ferrule 117 is held by the pipe 115 and airtightly fixed at a tip of the pipe 115.
The optical waveguide device 110 described above is constructed by inserting the optical fiber 113 with the first ferrule 116 and the second ferrule 117 provided on the optical fiber 113 (the optical fiber 113 with the first ferrule 116 and the second ferrule 117 positioned on the optical fiber 113) through the pipe 115 airtightly fixed to the package 112 and connecting the first ferrule 116 to the optical waveguide chip 111.
At this point, the connection structure of an optical fiber, such as material selection and dimensions of each element, is designed under conditions of [1] matching thermal expansion, [2] securing an allowable bending radius of the optical fiber, [3] securing hermetic sealing, and [4] securing nose strength.
Here, concerning [1] matching thermal expansion, heat is added in a manufacturing process to an optical waveguide device in which the optical fiber 113 is connected to the optical waveguide chip 111 mounted in the package 112 to cause an adhesive used for connection to set, and material and dimensions of each part are designed in such a way that a first amount of expansion or contraction caused by thermal expansion of part a of the package 112 in FIG. 14 and thermal expansion of part b of the pipe 115, and a second amount of expansion or contraction caused by thermal expansion of part c of the optical fiber 113 and thermal expansion of part d of the second ferrule 117 match. No tensile stress or compressive stress caused by thermal expansion or contraction will thereby be applied to a connection (fixed) part of the optical waveguide chip 111 and the optical fiber 113 by thermal expansion or contraction to stabilize conduction characteristics of the optical fiber 113 even if heat is added.
Also, concerning [2] securing an allowable bending radius of the optical fiber, the bending radius of the optical fiber 113 is secured by setting member accuracy so that desired relative position accuracy of a position (an adhesion position with the optical waveguide) of the first ferrule 116 and that of a position (a position of the pipe 115 of the package 112) of the second ferrule 117 are attained to suppress a loss by bending of the optical fiber 113 to allowable limits.
That is, it is desired that a portion of the optical fiber 113 between the first ferrule 116 and the second ferrule 117 where the optical fiber 113 itself is exposed is in a straight line if possible. However, it is difficult to maintain the optical fiber 113 in a straight line based on accuracy of a positional relationship when the first ferrule 116 and the second ferrule 117 are provided on the optical fiber 113 and accuracy of a positional relationship of the optical waveguide chip 111 mounted in the package 112 relative to the pipe 115, and therefore a loss is controlled to allowable limits by making the exposed portion of the optical fiber 113 longer to some extent so that, even if the optical fiber 113 is bent, a bending angle of the optical fiber 113 becomes greater.
Further, concerning [3] securing hermetic sealing, hermetic sealing of the package 112 is secured by airtightly fixing a portion (See a portion B in FIG. 14) where the optical fiber 113 and the second ferrule 117 are fixed, a portion (See a portion C in FIG. 14) where the package 112 and the pipe 115 are fixed, and a portion (See a portion A in FIG. 14) where the tip of the pipe 115 and the second ferrule 117 are fixed to stabilize operations of the mounted optical waveguide chip 111.
Also, concerning [4] securing nose strength, nose strength is secured by firmly fixing the pipe through blazing of the package 112 and the pipe 115 and inserting the second ferrule 117 into the pipe 115 substantially without gap to hold the second ferrule 117 by the pipe 115.
As additional conventional techniques related to the present invention, those described in Patent Document 2 to Patent Document 4 are known.
[Patent Document 1] Japanese Patent Laid-Open No. 2006-119373
[Patent Document 2] Japanese Patent Laid-Open No. 2001-154064
[Patent Document 3] Japanese Patent Laid-Open No. 2000-352644
[Patent Document 4] Japanese Patent Laid-Open No. 2001-281498
Nevertheless, problems shown below have inhibited the realization of the above-mentioned (a) miniaturization by making an optical fiber shorter and (b) advanced features by making an optical fiber multi-core (multi-channel) in the above-mentioned optical waveguide device 110 shown in FIG. 14.
That is, the position of the first ferrule 116 and that of the second ferrule 117 are determined by the position of the optical waveguide chip 111 and that of the pipe 115 respectively in the optical waveguide device 110 shown in FIG. 14, but if there is any deviation of positions of both, the optical fiber 113 will be bent (deflected).
Making the length of the above-described exposed portion of the optical fiber 113 shorter as much as possible contributes to miniaturization of the package 112, and eventually to miniaturization of the optical waveguide device 110. However, as described above, bending (deflection) of the optical fiber 113 caused by a deviation of the positional relationship when the first ferrule 116 and the second ferrule 117 are provided on the optical fiber 113 from that of the optical waveguide chip 111 mounted in the package 112 relative to the pipe 115 will be larger as the length of the exposed portion of the optical fiber 113 becomes shorter even if the deviation is slight.
Therefore, at least the positions of the optical waveguide chip and pipe must be determined with very high accuracy to make the optical fiber 113 shorter for miniaturization of a package 112 scale and to secure the allowable bending radius. However, since there is naturally a limit to dimensional accuracy of components and positioning accuracy of assembly, there has also been a limit when making the optical fiber 113 shorter (that is, miniaturization of the package 112).
Further, if, when the optical fiber is made multi-core, a deviation in a tilt (oscillation) direction with respect to an optical axis is added to a relative position of the first ferrule 116 and the second ferrule 117, the optical fiber 113 is easily buckled (pushed down). To suppress “bending (deflection) by buckling” to make the optical fiber have a radius equal to or larger than the allowable bending radius, the tilt (oscillation) direction of the first ferrule 116 and the second ferrule 117 must be positioned with high accuracy.
However, there has naturally been a limit when pursuing very high accuracy on component dimensions and assembly, which are factors of the tilt (oscillation) such as an end face angle of the first ferrule 116 and an inclination of the pipe 115. Thus, both of the above-described “bending (deflection) by position deviation” and “bending (deflection) by buckling” must be considered to make the optical fiber 113 multi-core.
Techniques described in Patent Document 2 to Patent Document 4 do not mention any technique that could make the optical fiber shorter by increasing positioning accuracy.